work around new import statements generated by swig-4.0

Those statements are not compatible with the fact that
libtool-generated modules are in .libs/ until they get installed.

* python/spot/__init__.py: Add sys.path to __path__ if SPOT_BUILD is
  set.
* tests/run.in: Set SPOT_BUILD.

python/spot/__init__.py

@@ -20,21 +20,27 @@
 
 import sys
 
-
 if sys.hexversion < 0x03030000:
     sys.exit("This module requires Python 3.3 or newer")
 
+import subprocess
+import os
+import signal
+import tempfile
+from contextlib import suppress as _supress
+
+if 'SPOT_UNINSTALLED' in os.environ:
+    # When Spot is installed, _impl.so will be in the same directory as
+    # spot/impl.py, however when we run Spot's test suite, Spot is not yet
+    # installed and we want "from . import _impl" (generated by Swig4) to look
+    # into .libs/
+    __path__.extend(sys.path)
 
 from spot.impl import *
 from spot.aux import \
      extend as _extend, \
      str_to_svg as _str_to_svg, \
      ostream_to_svg as _ostream_to_svg
-import subprocess
-import os
-import signal
-import tempfile
-from contextlib import suppress as _supress
 
 
 # The parrameters used by default when show() is called on an automaton.